KOTIFBRA. 403 



the form of prominences beset with hairs and seta 1 , or even of tubular 

 elongated processes of the skin (respiratory organs of the neck), 

 beneath which the sensory nerves end in ganglionic swellings. 



Generative organs. The sexes are separate, and are distinguished 

 by a strongly marked dimorphism. The very small males have 

 neither oesophagus nor intestinal canal, which are reduced to a string- 

 like rudiment; and they leave the egg completely developed. Tlu-ir 

 generative organs are reduced to a testicular sac filled with 

 spermatozoa, the muscular duct of which opens at the hinder end 

 of the body, sometimes on a papilliforni protuberance. The generative 

 organs of the females, which are far larger than the males, consist of 

 a roundish ovary filled with developing ova, and of a short oviduct 

 which contains one or but few ripe ova, and usually opens into the 

 cloaca, Almost all Rotifera are oviparous; and their eggs are 

 distinguishable into thin-shelled summer eyys and thick-shelled 

 winter eggs. They carry both kinds of eggs about on their body, 

 but the summer eggs not unfrequently undergo their embryonic 

 development in the oviduct. The summer eggs probably develop 

 parthogenetically, since at the season of the year when they appear 

 the males are not to be found. The thick-shelled winter eggs, 

 which are often dark coloured, are produced in the autumn and 

 fertilized. 



Development. As far as the embryonic development is known, it 

 shows a great agreement with that of many Gasteropoda (Calyptrceci). 

 The ova undergo an irregular segmentation. The cells proceeding 

 from the smaller segmentation spheres become accumulated at one 

 pole, and finally enclose the darker coloured yolk cells completely, so 

 that a two-layered embryo is formed. The cells of the outer layer 

 are much poorer in granules than are those of the central entoderm 

 layer, and form the ectoderm. A depression of the ectoderm is 

 formed on the (later) ventral surface, from the side walls of which 

 the two lobes of the trochal disc grow out (like the oral lobes of 

 mollusc embryos). The hinder portion of the depression becomes 

 the posterior part of the body, at the base of which a pit forming 

 the first rudiment of the cloaca makes its appearance. The mouth 

 and the anterior part of the alimentary canal are developed anteriorly 

 at the bottom of the depression. The ganglion arises from the 

 ectoderm in the cephalic region. There are no reliable observations 

 on the formation of the mesoblast. In the male embryo the 

 development takes a different course, the alimentary canal not being 

 completely developed. The free development takes place either 



